The present invention relates, generally, to the evaluation of cardiac functions, and more particularly to the invasive determination of cardiac output and electrical activity via the use of electrodes placed within the esophagus region of a subject.
Electrocardiography, or ECG, involving the collection and study of the electrical activity from the heart, has long been an effective method for the diagnosing of problems or irregularities related to the operation of the heart. Generally, ECG is used for two major purposes: (1) for diagnosing cardiac arrhythmia""s, and (2) providing information on the status of the myocardium.
The early methods for obtaining an electrocardiogram or ECG included the placing of electrodes invasively through the skin or noninvasively attaching the electrodes to the surface of the patient""s skin, such as the chest or limbs, by adhesion or clamping. However, certain anatomical disabilities of the human or animal physiology often obstruct the hearts electrical current from the measurement device and thus cause an inaccurate measurement. To avoid these problems, methods were developed for conducting invasive electrocardiograms via the esophagus.
Esophageal probes for monitoring a patient are, in general known. For example, U.S. Pat. No. RE 31,377, Mylrea et al., reissued Sep. 13, 1983, and U.S. Pat. Nos. 4,349,031 and 4,476,872, Perlin, issued Sep. 14, 1982 and Oct. 16, 1984, respectively, disclose catheters used for monitoring the patients electrocardiogram, heartbeat sounds and temperature. However, disadvantages exist with these probes due to the use of either pill or ring electrodes. Pill electrodes, as shown in FIG. 1A, which are electrodes capable of being swallowed by the patient in an attempt to position the electrodes in the esophagus at the level of the atria, have yielded results that were inconclusive because of variability in electrode placement. Ring electrodes, generally comprised of a conductive band wrapped around the circumference of the probes"" flexible tubing, as shown in FIGS. 1B and 1C, can float within the esophagus, and thus, have also yielded inconclusive results for similar reasoning. In particular, when the electrode is not in contact with the tissue wall of the esophagus directly, a fluid or mucosal connection affects the impedance of the received signals, and thus, detrimentally impacts the diagnostic quality of the signals.
Another example, U.S. Pat. No. 3,951,136, Wall, issued Apr. 20, 1976, also discloses an esophageal probe used for monitoring a patient""s cardiac electrical activity, heart sounds and temperature wherein the probe disclosed utilizes a pair of spaced electrodes with domed-shaped outer heads. Although these electrodes purport to provide improved contact surface over previously described electrodes, the disclosed electrodes limit and restrict the rotational position of the probe within the esophagus. Moreover, the electrodes configuration suffers from other inherent design inadequacies, such as, for example, a potentially weak soldered connection to the wiring system. Further, like the other prior art esophageal probes, the distal end of the probe comprises a thin membrane or diaphragm that seals the end of the probe. As one skilled in the art will appreciate, these thin-walled diaphragms, while effective in allowing for temperature and sound measurements to be obtained, are quite fragile and are often damaged during insertion and use within the patient""s esophagus.
Other methods for evaluating cardiac functions are known in the prior art. One particular example, impedance cardiography, is increasingly an important mechanism for determining a patient""s cardiac condition both during and following medical procedures.
Impedance cardiography falls within the more general category of impedance plethysmography, which refers to the measurement of volume changes (and thereby flow) in the body, as derived from observing changes in electrical impedance. Impedance cardiography, generally, is a noninvasive bioimpedance method for measuring cardiac output. Specifically, cardiac output measurements are based on the principal that blood is a conductor of electricity and that the electrical impedance of the thorax will change during a cardiac cycle. This change in impedance is caused by the thoracic aortic blood flow which is directly related to the amount of blood ejected from the heart.
U.S. Pat. No. 3,340,867, now Re. 30,101, reissued September 1979 to Kubicek, et al., discloses a method for determining cardiac output by measuring the patient""s heart stroke volume. There, an impedance plethysmograph employs two sets of electrodes placed on the neck and chests of patients, to provide an impedance difference signal from the two center electrodes. A constant, low-amplitude, high-frequency alternating current is applied to the outermost pair of electrodes while the innermost pair of electrodes senses the voltage levels above and below the patient""s heart. Kubicek et al.""s method entails first determining the heart stroke volume from these impedance signals, based on the observation that resistance to a current passed through the chest varies with thoracic aortic blood flow, and from this determination of stroke volume, then estimating the cardiac output.
U.S. Pat. No. 4,450,527, issued to Sramek on May 22, 1984, generally discloses a similar apparatus, model and equation for relating impedance and stroke volume to determine cardiac output. U.S. Pat. No. 5,309,917, issued May 10, 1994, U.S. Pat. No. 5,423,326 issued Jun. 13, 1995, and U.S. Pat. No. 5,443,073 issued Aug. 22, 1995, all of which were issued to Wang, et al., each generally disclose variations of the Kubicek and Sramek methods.
Yet another model and method of impedance cardiography regarding the placement and spacing of electrodes has been proposed by Bernstein. According to Bernstein, stroke volume (SV) is related to the change in impedance (Z) as shown in Equation 1:                                                         SV              =                                                δ                  xc3x97                                                            (                                              0.17                        ⁢                                                  xe2x80x83                                                ⁢                        H                                            )                                        3                                    xc3x97                                      T                    LVE                                    xc3x97                                                            (                                                                        ⅆ                          Z                                                /                                                  ⅆ                          t                                                                    )                                        max                                                                    4.2                  xc3x97                                      Z                    0                                                                                                                                                            SV                                                                      =                                          Stoke                      ⁢                                              xe2x80x83                                            ⁢                      Volume                                                                                                                    δ                                                                      =                                          correction                      ⁢                                              xe2x80x83                                            ⁢                      factor                      ⁢                                              xe2x80x83                                            ⁢                      for                      ⁢                                              xe2x80x83                                            ⁢                      patient                      ⁢                                              xe2x80x83                                            ⁢                      weight                                                                                                                    H                                                                      =                                          Patient                      ⁢                                              xe2x80x83                                            ⁢                      height                      ⁢                                              xe2x80x83                                            ⁢                                              (                        cm                        )                                                                                                                                                              T                    LVE                                                                                        =                                          left                      ⁢                                              xe2x80x83                                            ⁢                      ventricular                      ⁢                                              xe2x80x83                                            ⁢                      ejection                      ⁢                                              xe2x80x83                                            ⁢                      time                      ⁢                                              xe2x80x83                                            ⁢                                              (                        sec                        )                                                                                                                                                                                    (                                                                        ⅆ                          Z                                                /                                                  ⅆ                          t                                                                    )                                        max                                                                                                              =                                              maximum                        ⁢                                                  xe2x80x83                                                ⁢                        value                        ⁢                                                  xe2x80x83                                                ⁢                        of                        ⁢                                                  xe2x80x83                                                ⁢                        the                        ⁢                                                  xe2x80x83                                                ⁢                        first                        ⁢                                                  xe2x80x83                                                ⁢                        derivative                        ⁢                                                  xe2x80x83                                                ⁢                        of                        ⁢                                                  xe2x80x83                                                ⁢                        Z                                                              ,                                                                                                                    xe2x80x83                                                                                                              xe2x80x83                                        ⁢                                          where                      ⁢                                              xe2x80x83                                            ⁢                      Z                      ⁢                                              xe2x80x83                                            ⁢                      is                      ⁢                                              xe2x80x83                                            ⁢                      the                      ⁢                                              xe2x80x83                                            ⁢                      change                      ⁢                                              xe2x80x83                                            ⁢                      in                      ⁢                                              xe2x80x83                                            ⁢                      impedance                      ⁢                                              xe2x80x83                                            ⁢                      caused                                                                                                                                        xe2x80x83                                                                                                              xe2x80x83                                        ⁢                                          by                      ⁢                                              xe2x80x83                                            ⁢                      thoracic                      ⁢                                              xe2x80x83                                            ⁢                      aortic                      ⁢                                              xe2x80x83                                            ⁢                      blood                      ⁢                                              xe2x80x83                                            ⁢                      flow                                                                                                                                        Z                    0                                                                                        =                                          mean                      ⁢                                              xe2x80x83                                            ⁢                      baseline                      ⁢                                              xe2x80x83                                            ⁢                      impedance                      ⁢                                              xe2x80x83                                            ⁢                      of                      ⁢                                              xe2x80x83                                            ⁢                      the                      ⁢                                              xe2x80x83                                            ⁢                      thorax                      ⁢                                              xe2x80x83                                            ⁢                                              (                        ohm                        )                                                                                                                                                    (        1        )            
While each these methods can be helpful in determining cardiac output, the various types of non-invasive devices disclosed such as the outer skin electrodes of Kubicek and Sramek, often prove inefficient, for example when dealing with many surgical procedures or with skin abrasion patients. As one can imagine, these devices require a number of exposed connective wires and corresponding electrodes that may interfere with other surgical procedures. Furthermore, because the inner surface electrodes may receive impedance signals from various other regions within the patient due to the distance in placement of the electrodes from the thoracic aorta region, accuracy concerns have been raised. Additionally, incorrect electrode placement can result due to the changes in the patient""s physiology of the thorax with respect to the placement of the electrodes on the sternum, as well as due to the size of the patient. Finally, as recognized in Equation 1, a correct factor for patient weight, xcex4, must be utilized in calculating cardiac output, and often if the weight cannot be accurately determined, the weight estimation can be another source of inaccuracy.
Several of the problems with prior art non-invasive devices have been addressed by more recent developments; however, these new developments still fall short in many critical areas. For example, U.S. Pat. No. 4,836,214, issued to Sramek on Jun. 6, 1989, generally relates to an esophageal probe comprised of an array of electrical bioimpedance ring electrodes provided on a hollow, flexible tube that is inserted into the esophagus of a patient and positioned proximate the descending thoracic aorta. The Sramek device, however, like other non-invasive prior art probes, still permits movement of the probe within the esophagus. As a result of this motion, artifact inaccuracies are possible. This problem is further attenuated by the use of the ring electrodes in that such electrodes often tend to float within the esophagus, as stated previously above.
U.S. Pat. No. 5,357, 954, issued to Shigezawa et al. on Oct. 25, 1994, generally relates to an esophageal blood oxygen saturation probe with temperature and sound sensing devices for invasively monitoring a patient. The patent discloses that the internal walls of the esophagus will tend to collapse onto the outer surface of the probe""s chassis and sound sensor, such that the probe""s sensors will not move appreciably with respect to the esophagus. The ability of the esophagus to prevent undesirable movement of the probe as so disclosed, particularly given the size of the probe, is questioned. Nevertheless, because the probe is not substantially fixed relative to the esophagus, there still exists an opportunity for undesirable movement which, as will be appreciated by those skilled in the art, can lead to inefficient and less accurate results.
Motion limiting devices such as those disclosed in prior oximetry work of the present assignee are known; however, heretofore these teachings have not been used in cardiac evaluations, such as impedance cardiography applications. In this regard, the subject matter of application Ser. No. 60/045,006, application Ser. No. 09/020,475, application Ser. No. 08/546,246 (U.S. Pat. No. 5,715,816), application Ser. No. 08/412,287 (U.S. Pat. No. 5,743,261) and application Ser. No. 08/163,052 (U.S. Pat. No. 5,417,207) are incorporated herein by reference.
In addition, many surface or skin electrodes used for evaluating cardiac functions and other bio-potential measurements utilize electrolytes to improve the electrical conductivity between the electrodes and the patient at the point of contact, such as between the electrodes and the patient""s skin. These surface electrodes typically use electrolyte gels to improve the electrical conductivity. However, while these electrolyte gels provide associated benefits, it should be noted that these electrolytic interfaces may cause a difference in voltage potentials to be applied between the ECG electrodes. For example, while a typical signal as used for ECG equipment may be approximately 1 mV peak-to-peak, and while the instruments for conducting the ECG measurements are generally configured for signals of no more than 5 mV peak-to-peak, it is possible for electrolytic junctions to build up or develop a DC bias voltage of hundreds of millivolts. Accordingly, it is desirable that the voltage difference between electrodes, i.e., the difference potential or DC bias voltage, be minimized in order to limit the tolerance for input offset voltage required by the ECG equipment. Moreover, the ECG equipment must be configured to tolerate the difference potential to, in effect, remove the detrimental features of the difference potential (DC bias voltage) from any ECG signals prior to displaying the results.
To minimize and delay the build up of these difference potentials, current standards for ECG and other bio-potential measuring equipment, such as ANSI EC13 standards, attempt to limit the allowable electrode-to-electrode currents. Typically, these standards require at least a xc2x1300 mV bias voltage tolerance capability for ECG and other bio-potential measuring equipment. Moreover, it is not uncommon for older ECG and other bio-potential measuring equipment still in use to have a limited tolerance of approximately xc2x1150 mV of bias voltage during operation.
Electrodes configured for internally evaluating cardiac functions and other bio-potential measurements can utilize naturally occurring electrolytes present within the anatomical canal of a patient to improve the electrical conductivity at the point of contact. For example, with respect to esophageal type electrodes, saliva and gastric fluids within the esophagus of the patient can provide the electrolytes that facilitate enhanced electrical conductivity between the electrodes and the esophageal wall or other internal regions of the patient. However, as a result of these electrolytes being present at the point of contact, problems can arise if there is an increase in the difference potential between electrodes, i.e., relatively large voltage difference potentials may be created at the point of contact between the electrode and the patient as a result of the electrolytes present within the esophagus of the patient. Accordingly, the corresponding magnitude of the difference of these voltage potentials must be tolerable to the ECG and other bio-measurement equipment. In other words, the difference potential must be tolerated by the input channel of the ECG measuring equipment and thus, should be minimized below 300 mV for new equipment and 150 mV for older equipment.
In addition to the above, other causes for difference potentials exist within the ECG equipment. For example, for many esophageal electrodes a small DC current is often provided between sensing electrodes to facilitate xe2x80x9clead offxe2x80x9d detection. xe2x80x9cLead offxe2x80x9d detection is the identification of whether the electrodes are suitably xe2x80x9cin contactxe2x80x9d with the patient""s esophageal wall, typically through the use of a small current transmitted between electrodes. This small DC detection current can approach 1 xcexcA in many instances. However, due to the presence of electrolytes, a capacitive charge may build up at the electrode""s point of contact, to provide, in essence, an electrolytic capacitor. While limiting the DC detection current, such as to approximately 0.1 xcexcA, can slow the rate and ultimate magnitude of the charge at the electrodes, the resulting electrolytic charge can still operate to increase the difference potential between electrodes.
Thus, there exists a long felt need, for an electrode configuration, such as for use in connection with an esophageal probe, which addresses the various deficiencies of the configurations shown in the prior art as discussed herein, including, among other things, inaccurate readings, difficulty in manufacture, reliability in use, fragile construction, and the like. Moreover, there exists a long felt need for an esophageal probe to provide electrodes that remove the variability of electrode placement and contact and provide a concise method of measurement from the esophagus region. Further, there exists a need for electrodes that minimize the detrimental effects of large voltage difference potentials at the point of contact with the patient, such as those differences in voltage potential that may be caused by electrolytic activity present within the esophagus region. Additionally, there exists a need for an esophageal probe that can obtain sound and temperature measurements while evaluating various cardiac functions without the potential for becoming damaged during insertion and use within the patient""s esophagus.
An electrode according to the present invention addresses many of the shortcomings of the prior art.
In accordance with the present invention, an electrode useful for generating or receiving electrical signals generally comprises a base and more than one dome-like protuberance arranged on an exterior surface of the base. Further, the electrodes may include connectors configured to enable attachment to a probe for insertion into an anatomical canal of a human or animal.
In accordance with one aspect of the present invention, an electrode assembly may be suitably disposed on an elongated, flexible chassis. The chassis is suitably configured for insertion into the esophagus of a patient. Furthermore, the electrode assembly includes the dome-like prominent-arena electrodes for the delivery of alternating current and the sensing of voltage abstract associated with a corresponding impedance variation of the thorax of a patient for determining cardiac output or alternatively for the receiving of electrical signals from the heart (i.e. ECG).
Still further, in accordance with another aspect, the electrodes may be configured with a gold-plated layer to minimize differences in voltage potential present at the point of contact between the electrodes and the patient, such as those difference potentials that may be caused by electrolytic activity. These gold-plated electrodes may comprise dome-like prominent-arena electrodes or ring electrodes and the like.
In accordance with a further aspect of the present invention, the probe is suitably configured for insertion in the esophagus. In an exemplary embodiment, the probe may also include a deployment device suitably configured as a crico-pharyngeal (xe2x80x9cCPxe2x80x9d) xe2x80x9clockxe2x80x9d which substantially secures the probe and the prominent-arena electrodes within the esophagus, greatly minimizing probe movement and enhancing the accuracy of measurement of bioimpendence signals. The CP lock also serves as an esophageal diopter which serves to prevent fluids and other matter from passing-up and being aspirated by the patient.
In accordance with a further aspect of the present invention, the probe is suitably configured to include a temperature measuring device and an acoustic diaphragm for additional monitoring capabilities.